Question

求解第11题

Answer 1

cos36-cos72
=cos(54-18)-cos(54+18)
=cos54cos18+sin54sin18 - [cos54cos18-sin54sin18]
=2sin54sin18
=2sin(90-36)sin18
=2cos36sin18
=2cos36sin18 cos18/cos18
=cos36sin36/cos18
=sin72/(2cos18)
=sin72/(2cos(90-72))
=sin72/(2sin72)
=1/2

Answer 2

cos36-cos72
=cos(54-18)-cos(54+18)
=cos54cos18+sin54sin18 - [cos54cos18-sin54sin18]
=2sin54sin18
=2sin(90-36)sin18
=2cos36sin18
=2cos36sin18 cos18/cos18
=cos36sin36/cos18
=sin72/(2cos18)
=sin72/(2cos(90-72))
=sin72/(2sin72)
=1/2
（°省略）